1. Field of the Invention
The invention relates to a method of inspecting a polycrystalline semiconductor film, and more particularly to a method of inspecting grain size of a polysilicon film.
2. Description of the Related Art
Currently, thin film transistor-liquid crystal display (TFT-LCD) technology mainly adopts two kinds of thin-film for fabricating transistors. One is amorphous silicon film, and the other is polysilicon film. The polysilicon thin film transistor (TFT) possesses the advantage of having electron mobility 10˜100 times as high as that of amorphous silicon TFT. Therefore, there has been studied and developed a drive circuit integrated TFT-LCD using polysilicon TFT as a pixel switching element or a peripheral drive circuit for an LCD.
Polysilicon TFT is fabricated by a low temperature polysilicon (LTPS) process. In the LPTS technology, a polysilicon film is formed by performing excimer laser annealing (ELA) on an amorphous silicon film. Since the process temperature is below 600° C., this technology can be applied to a transparent glass substrate. In general, the electron mobility of polysilicon TFT is dependent on the grain size of a polysilicon film. That is, the electron mobility of polysilicon TFT is increased by increasing the grain size of a polysilicon film. In addition, the grain size of a polysilicon film is related to the laser energy density applied to the amorphous silicon film. Accordingly, it is necessary to measure the grain size of a polysilicon film, thereby determining an optimal laser energy density to control its grain size.
Conventionally, there is a well known method for observing the surface roughness of a polysilicon film using an optical microscope (OM) with a magnification of approximately 500˜1000 to serve as a grain size index. However, since this method relies mainly on the human eye, the measured result is imprecise. Another conventional inspection method adopts a scanning electron microscope (SEM) to measure the grain size of a polysilicon film. This method, however, a destructive and excessive time is spent on sample fabrication and inspection, thereby decreasing throughput. In order to reduce the inspection time, use of an atomic force microscope (AFM) has been proposed. Although grain size can be observed and measured by AFM, approximately 30 minutes is required to observe and analyze one point of grain size. That is, AFM is unsuitable for analyzing multi-points.
Recently, use of an ellipsometer for measuring and use of effective medium approximation (EMA) or dispersion law for spectrum regression has been proposed. This method, however, cannot precisely obtain an optimal laser energy density and crystalline ratio, especially when the polysilicon film is formed by super lateral growth (SLG).